Field of the Invention
The invention relates to a method for determining an extension of an object by means of an extensometer having two legs which are each connected at one end to a joint, are movable in a spreading plane and are each connected at the other end to the object, so that the extension spreads the legs apart. The invention furthermore relates to an extensometer for carrying out the method.
Extensometers of the type mentioned above have two usually approximately straight legs which are each connected at one end to a joint, are movable in a spreading plane and are each connected at the other end to the object. The spreading plane is oriented essentially perpendicularly to a surface of the object between the other ends of the legs, that is to say between the connection points of the legs to the object. The extensometer therefore "stands" on the surface in the manner of a step ladder.
Such extensometers and methods for using them are known from European Patent No. 0 088 278 B1, German Published, Non-Prosecuted Application DE 32 39 877 A1 and German Published, Non-Prosecuted Application DE 34 10 840 A1. The methods or the modes of operation of the extensometers are based on the fact that the two legs of the extensometers are fastened at their free ends to points of the object to be monitored which are distant from one another in the direction of the extension to be determined and are spread when the extension occurs. The legs are connected to one another at their other ends by means of a joint. As a result of the spreading of the legs, for example, the spacing between two capacitor plates, each of which is fastened to one of the legs, and therefore the capacitance of the capacitor configuration formed by the capacitor plates, is varied. The change in capacitance is in a fixed relation to the spreading of the legs and therefore in a fixed relation to the extension.
The known extensometers allow only the detection of extensions within a range of some tenths of a millimeter, and furthermore presuppose that the two fastening points of the legs move away from one another essentially only in a straight line when the extension occurs. A rotation of the legs relative to one another, as often happens when a load is exerted on fracture samples of the customary kind, is impossible or leads to disturbing torsional loads on the extensometer. Moreover, because of the thickness of conventional capacitor plates, the minimum spacing of the legs amounts to several millimeters. That can be unfavorable if the detection of extensions is to take place between two points which are initially located closely adjacent one another, for example on both sides of a crack, the widening speed of which is to be investigated.
German Utility Model G 88 03 392 U1 discloses a displacement sensor having two legs which can be applied to a material sample and which have, at their free ends, pick-up elements, by which the displacement sensor is fixed to the material sample. A measured quantity proportional to a change in displacement is supplied through at least one strain gauge fastened to the displacement sensor. The legs of the displacement sensor are formed of resiliently elastic steel. They are fastened to the material sample in such a way that spreading occurs in a spreading plane parallel to the surface of the material sample.
European Patent No. 0 088 278 B1 discloses an extensometer having two legs which are each connected at one end to a joint and are each to be connected at the other end to an object to be investigated. The legs stand on the object. In other words, a spreading plane defined by the spreading of the legs is perpendicular to the surface of the object. In order to fasten them to the object, each leg has holding strips which are laid onto the object.